I feel like the original language of Lunge was supposed to be for the full round.
I read Monkey Lunge and it seems to me like they are saying "give up your standard action and get to make AoO at +5' without AC penalties".
I read Mythic Lunge and it seems to me like they are saying "Expend a mythic power and negate a -2 penalty and get a +2 to AoO."
Now, it could be wishful thinking. But it could also be bad writing.
I see 3 charms. 2 imply that Lunge should interact with AoOs. 1 says it doesn't.
Either the chain for lunge is incredibly narrow, the two authors of Monkey Lunge and Mythic Lunge didn't understand how Lunge works. Or Lunge's language doesn't clearly reflect how the feat should work.
I think it's the latter but, without an FAQ clarification, I concede that it is also the weakest argument.
So let's see if it is too powerful.
Enlarge person is a level 1 spell. It gives you +5' reach. +2 Str. -2 Dex. and a few other effects for several minutes.
Lunge (liberally interpreted) gives you +5' reach for -2 AC (similar to -4 Dex) and for a turn.
Seem fairly even to me. So is Lunge (liberally interpreted) over powered?